Direct data modulation and detection of optical data signals at bit rates of 80-160 Gb/s and beyond is constrained by the modulation bandwidth of electro-optic devices, which are conventionally less than 40 GHz. A possible solution is optical time-division multiplexing (OTDM) whereby multiple 40 Gb/s channels of the same wavelength are temporally interleaved to form the higher bit rate signal [1].
At the receiver, electronic detection and bit error rate measurement are enabled by demultiplexing the OTDM signal to its constituent sub-rate channels. Typically, this can be done by fast optical gating to extract each sub-rate channel pulse using either an electro-optic device such as an electro-absorption modulator [2-4], or all-optical schemes involving interaction in nonlinear medium with co-propagating synchronized pump pulse [5-7]. While the electro-absorption modulator is simple to implement, demultiplexing performance for standard devices is inferior at bit rates of 160 Gb/s due to inadequate switching time.
Recently, a new scheme for high performance demultiplexing a 160 Gb/s signal to 40 Gb/s (i.e. 4:1 demux) was demonstrated using an a LiNbO3 phase modulator in conjunction with post-optical filtering [8]. It was shown that 40 GHz modulation of the 160 Gb/s signal manipulates the phase change over consecutive pulses in such a way to allow a 40 Gb/s channel to be discriminated by optical filtering. By similar principle, 2:1 demultplexing of 100 Gb/s signal has been shown using a phase modulator connected in a fiber loop [9], however in contrast to the filtering technique, this method requires cascading two modulators driven at different harmonic frequencies to enable equivalent 4:1 demultiplexing. Directly cascading LiNbO3 amplitude modulators for demultiplexing has also been reported whereby the delay between modulators is adjusted so that the combined switching response produces a shorter gating window [10].